Yes it’s true that it’s not healthy to have a relationship right after rehab.

I’m glad you care for him.
You should warn him , just remind him, if he feels strong for this girl.
It could make him happier.

Always be by his side. No matter what.

well it isnt the best idea to get directly into a relationship following rehab for the following reasons.

Someone who just got clean must change everything about who they used to be. They have to learn a new way of doing things, dealing with emotions and stress, new activities …. new self. If you go directly to a relationship all of those things may be defined by the relationship. They feel better and deal with emotions because they are with someone. They do activities that are new with their partner. So what happens when and if the relationship breaks up? First life was dealt with through drugs then when clean through the partner when the person suddenly is alone again they may relapse because they built their sober life around and through their partner

so it is important that if they do get involved they MUST have a life activities and time alone to develop themselves. A life outside of the relationship so they can cope with life regardless what happens to the relationship

secondly fresh recovery takes a lot of time energy and focus. A new relationship does as well. And it is often more fun than sobriety activities are. A person in a relationship in the beginning of their sobriety tends to choose relationship over aa na and other activities when sobriety should come first. When they stop showing up for sobriety and start focusing on the relationship it is likely they will relapse.

If they get into a relationship their partner MUST allow them and encourage them to put their sobriety and all sobriety activities first and their relationship and time with their newly sober partner second. If the partner is unwilling to do this then it is NOT a relationship that a newly sober individual should be in.