You can't force parents out of denial. The best you can do is gentle prod them towards reality. Continue to work with the child as best you can.

Either the child is just catered to at home and she will eventually catch up, or she is in fact delayed and needs extra help. You can't force the parents to seek that help but can do your best on your end. Use gentle statements to express what you observe but don't be so overt and firm that they shut down.

13 months is still young enough that she can catch up with the right nudging on your part. If she just has no need to roll on her own she won't learn to...it is hard work, who wants to do it when mom and dad will do it for me. I have had a 13 month old start that couldn't support her own weight on her legs. Because she NEVER had to she was carried. Within 4 weeks she was walking...because she saw the other kids doing it and suddenly realized she wanted to. The parents just truly never felt the need to let her. No harm done, the child walks perfectly normal. She just needed to want to.

I would suggest taking notes. Write your observations, note any improvements and great areas of struggle. It may come in handy in the future if the family comes to see a problem.