I haven't been in this type of relationship, but my very good friend has been in a few. What I noticed was that although she was in the long-term looking for a long lasting relationship and only having fwb in the meantime until she met someone else. It never ended up happening. Perna touched on this already.

Since she was in fwb relationships it somehow closed the door a bit to meeting other people. In the end she decided this wasn't helping her find a long term partner. She also went through a lot of heartche as people move on and emotions got in the way.

I think it can work for some people, some of the time. But you mentioned your friend may have more feelings involved than you. I would take caution with that knowledge.