Yes, there is an excellent option from the MELT method. I found a link that shows a picture and a detailed description of the “Bent Knee Press”. http://www.meltmethod.com/node/3452. Please note that your client may hold the knee also behind the thigh rather than the knee. Even a half roller will do even though it is not as effective. Use the softest roller you have for it, of curse preferably the MELT roller.
I assume that the client can get on the floor. Else, you may need to use a massage table or wider as used in Feldenkrais.
Good luck. Try it for yourself, you may never do the traditional quadriceps stretch again.