When you observe that an aircraft appears to be in an unsafe proximity to terrain, obstructions, or traffic, issue a safety alert. Use phraseology similar to the following:

“low altitude alert, check your altitude immediately, the MVA in your area is one zero thousand five hundred.”
“traffic alert, traffic twelve o'clock, one mile, maneuvering, altitude indicates seven thousand five hundred.”

If appropriate, you may also issue an alternate course of action, using phraseology similar to:

“advise you turn left heading two two zero”
“advise you climb five hundred feet”

After you've issued a safety alert, it is up to the pilot to decide on the appropriate action.

{Reference: FAAO 7110.65 2-1-6}