T.H.
My niece was put into a residential treatment facility (more than once actually) and I will say this...was it helpful? Yes. Was it everything I hoped it would be? No. She didn't make the progress that we had hoped for, but she made meaningful connections, she tried really hard (sometimes!;) and it's somewhat comforting for the adults involved to feel like things are a little more controlled.
I think it's probably a good idea b/c they do a lot of therapy, a lot of problem solving, a lot of planning, coping strategies, etc. Is he in therapy now? If he is, he may be used to this routine. If he's not, this could really help him.
Is he on medication? Has it been recently changed? If it has, then I absolutely recommend a treatment center even more. A crazy statistic of suicide completions happen within weeks of a medication change. I don't know the actual number, but it's high.
One of the biggest cons of places like that however is, in a word, the population. The other kids in there will range from being first timers to frequent flyers. The other kids will also have "rap sheets" with minor stuff to being expelled from school, arrests, etc. He is probably going to learn some stuff she absolutely doesn't want him to learn, no matter how troubled he has been up until this point. That isn't to say it's not worth it, it's just more of a warning to be prepared.
This is a scary time for her and I truly, truly hope her son gets the help he needs.
ETA...Oh I also wanted to add that if she needs some help the hospital should have some sort of social worker to help her with this information so she isn't going at it alone. They can really help her figure things out.