The trim wedges is great advice. If that doesn't do the trick you may have to resort to a foil.
As I have posted before that would not be my first choice. A foil can dramatically improve handling, but I've had instances where the foil has taken of the top speed and turning radius of the boat.
I think the key is to find the smallest fin available for your motor. They are hard to find, but I believe Atwood makes a small 2 piece fin that might work well for you. The ones I'm talking about are shaped with a bend inverted upward towards the tips of the fin. Once you are up on plane most of the fin isn't in the water. Less drag to reduce your speed and turning radius is supposed to be the idea of that type of design. I belive they make that model in two sizes, look for the smallest one for your 25hp motor.
Please don't bother posting about how you've never experienced any negative results from a fin (as happens every time I post this). I have experienced loss of speed and turning radius in several installs over the years, so it does happen. I would not post it, if I had not experienced that personally on several install over the years.
With a 25 on a lifetimer, which is a fairly heavy aluminum boat for the size I do not think he would want to sacrifice any speed. That is why I'm suggesting that he look for the smallest fin with the least amount of drag that he can find.
Just because this has never happened on your installation does not mean that a speed reduction from installing a fin does not occur. I'm trying to make sure he avoids this problem with his install, if he chooses to go that route.