As a rule we are not here to tell you what you should or should not do.
Now... 15 hits is his normal limit, I have no idea what that means. One possibility is that said LSD is very very weak, another possibility is that your friend is really hardheaded. It's impossible to tell from an internet board point of view.
One important question is: how much acid does your friend take? If it's a whole lot then he might have tolerance to consider which you do not have obviously. Furthermore I wonder why is your buddy not able to predict for you what a good dose to start with is? He should consider this batch and ask himself: without any tolerance, what would be a nice threshold dose to get some nice relaxed moderate effect but not too much?
Remember that you can take more another, second time but you can't take back what you have already imbibed! There is no hurry so take it easy and don't put too much pressure on it. Otherwise there is only something to live up to, it's not worth going too far too fast.
About what to expect, we got miles and miles of threads about that.
Mushrooms and DMT are good things to have under your belt before starting, but personally I'd say that LSD is less mind-foggy or dream-like and more transparent rational and clear-headed.
Starting with half, then taking the other half later is hardly ever a bad plan... but realize that you don't have much to go on in terms of how strong a hit is. It is not a measure of quantity unless it's analyzed. Perhaps half a hit is already far too much (sincerely doubting that in this story) or a whole hit is far too little. It's all possible. To check that you would need time to start low, then double the dose when you repeat the test and so forth. That's called titration.
So back to basics you need to use the best info you have, which is what kind of dose has what kind of effect on a person? Ideally this would be an LSD-naieve person but okay... if you know anything about the batch like: start which such and such, maybe take half of that to be more sure and see what happens.