Holding a cup in a position to drink out of it, can be difficult for some people. To give some independence there are various solutions available, if someone is drinking at a table. However for mobile options, when it is required to mount a cup from a wheelchair, there are less choices available. The Mobilia mounting system used to offer a good cup holder, however this mounting system is no longer being manufactured.
After doing some searching, I found these two alternatives.
Rammount Cup holder
Rammounts is a well-established mounting systems for portable devices into cars. Their range of products include clamps that can fit around circular tubing. These clamps will fit onto a wheelchair frame. Also part of their range of components they have a drinks cup holder. Using various other components such as their double ball sockets and bars you can customise a mount to suit anyone. An advantage with this cup holder is that it remains level. So if its mounted to a wheelchair that tilts back, the cup should remain level.
Drinkup Travel Lite
Another option is Drinkup Travel Lite. It comes with a long straw that can be bent into position. The straw uses a non-return valve so that the liquid does not fall back into the cup if the liquid is only sucked up partially. This gives the opportunity to take only small sips at a time.