For push-ups I would say to keep doing them. If they can only do 5 - 6, let them have a break and then 5 minutes later try again. Do 3 - 5 sets and there'd definitely be improvement. It's consistency here.
For Pull-ups, a great prerequisite is negatives. Hold yourself up the top of the bar (where'd you'd be at the top of a pull-up) and lower yourself down in a controlled fashion. You can also do pull-ups in an inclined position (with feet still on the ground) holding onto rings, TRX or a bar.
For dips you can do them with your feet still on the ground and in addition use the triceps push-down machine at the gym if you have access to one.
In general, if your friend is overweight, they should lose it as any excess weight just makes bodyweight skills harder.