Well, it is obvious that the person wants a portrait that relates to this girl being a gymnast. Now how about doing a montage?
Use this or a similar action picture as a minor element, and then another image for the likeness. Since she is not available to pose at all ask for other photographs, preferably professionally taken that you might use even if she is not in her gymnastics uniform. For an example of what I am referring to, see Dick Bobnick's site and some of the sports figures he has done portraits of.
Link to his site at SOG
http://www.portraitartist.com/bobnick/bobnick.htm
It is always a challenge to do a portrait from photos alone and the posthumous portrait is perhaps the hardest. Getting the best photos you can is key to successfully pulling it off. I would not commit to doing the painting until you had all the reference you need to feel comfortable that you can complete the painting to a standard you will be happy to put your name on. Not just now but in the future. You do not want to look back on it and regret it is still around as a example of your work. Remember it is not your client's name that will go on this painting for eternity it is yours and once you deliver it, it is out there as a example of your work for the rest of your life and beyond. So yes, take the commission, but insist on getting more reference and if they really want the best painting from you they should give you what you need to make that possible.