Youre right on many levels, that said ,in some you are also in "ideal world" territory and that unfortunately is very rarely the case here. As I mentioned earlier in this , he knows her best, and knows how she will probably react etc and thats the missing info for us standing on the outside of this.
She may well be a great Mum close to her child, she may also be a "I got a life lead, kid is too young I aint doing this alone" Onlly the OP can assess that one.He's in a tough place and I wish him well with his choices.