If she is under maybe 35 years old, then I vote no. The kid doesn't need to see a string of different women running in and out of his life. He's already lost a mom - isn't that enough?
Also, dealing with other people's children is not a task to be taking lightly. It is a REALLY hard thing to do. I would never advise it for someone who has other options, and if she is in her 20s, she should have plenty of other options.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?