Is she eating too much!?

[mimi]

I would start introducing solids as well. I wouldn't want to keep feeding on demand plus you don't want to have the child associate food with comfort when in fact all she may need is her soother.
My daughter was a skinny premee who became a chubby toddler though she ate a mainly vegan diet and didn't have sugar till her first birthday cake so sometimes it is genetics (her dad was a chubby baby too)

[Fun&care]

When my kids were that age I was breastfeeding and they were fed every 1 1/2 hours, on demand. It was like that for both of them, yet one was always really skinny and the other had rolls all over I would say that that is her "normal". However she IS getting closer to 6 months and will be eating solids soon anyways. I think you should bring it up with the parents, and let them decide when to start solids. HTH!