In spoken English, I would recommend they/them/their, or sometimes he.
You should (almost) never refer to a person as it. Doing so indicates you do not consider that person to be human. In my experience, it may acceptably be used of a baby, probably precisely because we do not perceive infants as people. (Infants are people, but our perception may not properly respect that.)
Traditionally, he may be used in all cases as a default gender. The problem is that the gender is not truly neutral. Take your "friend" example:
Speaker 1: A friend of mine helped me a lot.
Speaker 2: You should thank him.
There have been so many times over the years where I have heard a conversation like this and automatically think, "Are you sure it was a him? Maybe it was a her," even though I know the generic he is being used! (I am a native speaker.)
So, although acceptable, the generic he will feel most awkward to English speakers when (in their own minds) it makes a real difference if the person being referred to is a man or a woman. Here's a rule of thumb. Imagine you found out if the person was a man or a woman. If that knowledge greatly changes your perception of the situation, then you should probably avoid using the generic he; otherwise, the generic he should be fine.
There may be other reasons not to use the generic he. It does have a bias toward the masculine, so that may get you into trouble with people who are sensitive to issues of sexism.
As a simpler alternative, use they/them/their instead, which are truly gender-neutral. They are perfectly acceptable in informal settings, and have also become acceptable in written English, as a way to avoid the disadvantages of other alternatives.
You can use they/them/their in almost all cases, but do be aware that these are technically plural, not singular. (Nevertheless, they are understood to be singular when used in this manner.) This has the potential to cause confusion in some cases, but unfortunately I can't think of any examples right now.
I would personally recommend against using he or she. This construction is fairly awkward in spoken English, and I think you will find it unwieldy in written English.
The alternatives he/she and s/he only apply to written English, and I would recommend against them as well. A trend in modern English is to simplify, minimizing the use of punctuation such as parentheses, periods in abbreviations, etc. Regular use of slashes is inconsistent with this goal.