Most don't realize why they foot fault.
The common advice is to tell the player just to back up.
Foot faults usually occur because a player does not squat down with their body like all good servers do, and Sampras does in the pics below:
In pic one, Pete has just released the ball. He is standing pretty erect.
But with his arm held high, he does a "squat", bending his knees deeply in pics 2- 9 into his trophy position.
In this deep squat, Pete's body weight is pushing him down - he has no choice but to push up hard with both feet at the same time.
That hard double leg push off will propel him "up the mountain" to help use his legs to power his serve.
On the other hand, many who have never had instruction in serving remain standing tall and just rock back onto their rear leg. Then when they start to bring their hitting arm forward, they have no choice but to step forward to keep their balance and in the process foot fault every time.
So if you foot fault, take a cue from Pete and squat down (bending your knees) while your tossing arm is held straight up.
Then use your leg push off to power your serve and to stop foot faulting.
Some might feel self conscious pointing this out to one who constantly foot faults.
And you have to tell them in a way without coming off as a know it all. (You can say you saw this tip on TT and think it really works.)
But you will be helping them a lot more than just telling to back up (which they may need to do for that match - but they have something to work on to really correct their problem in future matches.)