First I would take a look at their mechanics, hand placement, movement pattern.
Is this a new situation or has it always been an issue?

Carl Paoli has great progressions to check in.
Push Up Progression Pt.1
Push Up Progression Pt.2
Push Up Progression Pt.3

Identify restrictions in full range of motion?
Make any changes in their technique/movement pattern?

Are they on a computer a long time at work? home?
Typing position may need adjustments for the situation to resolve.