My sister-in-law was a single mom before marrying my brother, and I think for him, the key was to be open to the idea of getting involved with the kids. He always told me that, at first, he didn't try to incorporate her kid into anything because, he didn't want to be pushy and he wanted her to know he (and her kid) could trust him before forming a good relationship. Once he gained her trust, he began to spend time with the kid, and the rest is history. I think you just have to be verbal about how you are willing to be a part of her child's life, but only as much as she want you to be.
It's important that your buddy can be someone she can trust. She may have experienced situation in the past where she depended on someone who was not trustworthy. He also needs to be open minded and accept/understand that her priority will be her child.