(For my own sanity: A is your cat, B is the guest cat)
Will it be awkward/bad for my cat after the guest returns the her owner? Or will she be relieved and will return to her previous self quickly ?
That depends entirely on their interaction.
- If A is afraid of B's presence, she'll be relieved when B leaves (even if it takes here a few days to understand that she's gone).
- If B really grows on A, A might miss her. But I doubt that it'll be to a degree of heart ache, I expect it to be more a case of looking for B for a while before realizing she's not there.
- If A and B are rather indifferent to each other, I doubt B's departure is going to have any effect.
Edit: Both cats are mixed breeds and somewhat shy individuals.
Breeds shouldn't matter too much, unless you've got two breeds with extremely different character traits.
Shyness can swing either way. It can be positive (neither cat is very assertive) or negative (very territorial cats).
I have an empty room which the guest cat will stay with its own things/furnitures.
That's a good start.
I know a great deal about introducing all kinds of cats. I might let them free inside the house unsupervised if everything goes well for the first couple of weeks.
Note that you might have a slight benefit here: if you and your friend have visited each other regularly, you have probably brought each other's cat's smells home.
Swapping smells is one of the better approaches to getting cats to accept each other, and since they may have smelled each other on you/your friend before, introductions might already have been made (partially).
Note that while I appreciate the answers containing scientific experiments and results; I'm mainly looking for anecdotal evidences.
1 We have two sister cats, about 16 months old at the time. Very shy, very quiet. We adopt a 5 month old cat (initially we were a rescue home to him but then we decided to keep him). They were introduced gradually, not much fuss occurred.
The girls were interested, as was he. Social friction began when they realized that he's incessantly playful (and full of energy), whereas the girls were not as active.
One sister, the one who never plays much, ended up getting him to back off (after he bothered her repeatedly, she'd chase him out of the room, but was not violent about it). He learned from that and they mostly ignore each other now (after 10 months of living together). He learned to not engage her, he eventually got the hint.
The other sister, who is prone to bursts of energetic playing, is happy enough to play with him but sometimes still has to get him to back off. While he's still not great at finding the line between acceptable and too much, he does listen when "told off". These two don't really ignore each other except just after he's been told to back off.
After a while, even the least active sister played with the others (very occasionally). I've seen them hunt spiders together and so on. It's a peaceful household, maybe a bit overly playful at times.
2 When I was a kid, we had two cats. A 9 year old and a kitten. The 9 year old was very sweet, but hated the kitten. She was violent to her, intentionally sat next to the food (because she knew the little one was too afraid to go near her), ... While she herself had been abused by a male cat for years, she reversed the roles when she met the kitten.
We decided to test if we could give the older one away to friends and get another second cat. So we put her upstairs; and got a friend's kitten to visit for a day. This was only a few weeks after we brought our kitten home.
Our kitten immediately imitated the older cat. The guest kitten was harmlessly interested in her, but she hissed at her if she came close, tried to be violent, and blocked her from using the litter box.
We decided to keep both our cats, since getting rid of one of them wouldn't even solve the issue anyway. While they never got along, they eventually learned to avoid each other.
The second anecdote isn't meant to scare you. It's a very extreme scenario, mostly caused by the older one's history of being abused (she trusted no other cats anymore). The point of the story is that you can't always accurately predict interactions between cats.
Worst case scenario, you've got separate rooms for them. Best case, you have two pets for a while. If you hadn't had the separate room option, I would have been apprehensive of taking the cat in. But given the availability of that room (an your apparent experience with cat introductions), you're as prepared as you're ever going to be.