Mleigh, A question that you may or may not have the answer to...is your MIL passive-aggressive? The reason I ask is could she have clipped your son's bangs that short to get you to actually cut your son's hair, i.e., like now?
I don't think an email or a text message is going to do the trick. You will need to face her and look her in the eye and advise her that your son is your son and that you will determine when he needs a haircut, not her or your FIL. Sometimes you have to face them and verbally state your position on such things. If you don't, she'll just ignore the written word and go on about her business until next time.
In fact, if I were in your shoes, I would have to point out that since this has happened, you may have to think about allowing your son to visit w/a parent being present in the future. That might shake the old gal up a bit and give her something to think about the next time she picks up the scissors.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
