To the best of my knowledge, you only get exp when YOU water trees, either your own or your friend's. I believe friend watering is 10 per day and 1 self water, so at most 11 per day
7 to 8 is 500 exp, 8 to 9 is 1000 exp, so that's 46 and 91 days respectively.

That doesn't really affect you that much, at least not as long as you still get to harvest daily.
There are two exp for watering tree:
harvest exp, this depends on others watering you. But there is a nice bit that it increases for high friendship level, so if you have a few of those, it's not hard to get to harvest daily (I believe, 4, 7, 10 are the level where it gains 1 extra) In any case, you can only harvest once daily, so as long as that's met, it doesn't matter what they do.
level exp, this only depends on YOUR action, so whether or not they are being lazy won't affect you.

The ONLY thing that might hurt you, if you get to harvest daily, from their laziness is if they are one of your highest level/friend level friend that you might send onto commissions missions. Friendship level affects the income, so maintaining a few high level + high friend level friends will increase your coin income. But this rarely applies because you only get to send your top 3.