Entirely seriously, look into sets of 'post surgery recovery pillows'.

They're usually made of something similar to a dense foam mattress, and they'll be in modular, arrange able shapes to make it easier for people with serious injuries to recover at home, in a supported, semi reclined position.

... You can rearrange these into shapes that support positions other than sleeping/sitting.

Usually, these are cheaper than ... basically the exact same things, but explicitly sold as 'sex aides' or what not.

Helps? Probably not. Helps it not hinder having a good fuck? Quite possibly, especially if you can find a flat surface at the right height so both parties can keep their legs straight.

If it were a problem for me, I'd probably also be looking at how supportive a sex swing can be.

Reverse cowgirl can be a good position if the penis-haver has joint pain, if action is restricted to the bed.