I don't remember May as being a very humid month. This is before the hurricane season so we aren't getting the tropical depressions, etc. that tend to increase the humidity. And May is not the hottest month, either.
Since you are clearly not a newbie, you should have known you were opening the door to anyone looking for a bit of fun when you asked "exactly" when does it become humid. I'm surprised there have not been more responses giving "exact" days, hours, minutes, and even seconds.
Your mother wants to come here in May,then come in May. It is as good a month as any and better than July and August. If she finds it humid some afternoon, she can take a nap in her air-conditioned room.
I've been down in January, May and August... I found it cool and rainy in January, great in May and hot n humid in August... But pretty much it's always great to me just being in DR.
From statistics on rainfall we have been keeping for the last four years on the North Coast, May on average is the 3rd least rainy month ! How that translates into Humidity I dont know but it is a start!