The best way to remedy the bird's decreased aggressiveness is to try to find the source of its aggressiveness. It could be due to the disappearance of his friend but the aggressiveness could also be due to his teenage period (it depends on his age) or the breeding season (depends on the season). Be careful that this is not for a medical reason, because when the bird is sick or in pain, it may show signs of aggression.
To remedy aggressive behavior we can start by making sure that the cage is big enough and in a well-located place where the bird sees family activity, that it can participate in family life, and that it does not do not feel excluded from the group, we can also place the cage near a window taking certain precautions such as the sun's rays on the glass or the cold and drafts. There is also the respect of the rest of the parrot, it must benefit from 12 hours of sleep per night and a nap of at least 30 minutes in the afternoon, a balanced diet, good hygiene of the plumage and the cage or the aviary, will allow the bird to regain good mental and physical health.
When he's aggressive towards you or another family member, don't react (although this can be difficult at times) because the slightest reaction from you can encourage his aggressive behavior. Use positive reinforcement to educate or re-educate your bird with patience and spending time with him. Positive reinforcement is a reward-based bird education method. The objective is to make the bird understand the relationship between the link and the reward. The bird will quickly associate good behavior with a reward. The reward fears, for example, a food treat that he likes or a caress, encouragement by the voice. It is a gentle method without punishment that reinforces any behavior. In the beginning, you have to systematically and immediately reward every positive behavior; subsequently, you can decrease the frequency of rewards and give them more randomly in order to maintain the acquired behavior.