If your friend puts in for the WHV, that person isn't going to be asked questions about the reasons for their visit in quite the same detail that someone on a visitor's visa would be. By definition, they're going to be on holiday, with the option of doing some work to support themself. INZ are not the thought police, or crystal ball users. The fact of meeting/staying with an internet-met friend is a reason for going to a particular place within the country, and knowing all about that area in advance, for tourist activities, and places to visit. For official purposes, leave it at that at the outset. If you find that all works out as you hope, and the chemistry is there when you meet in person, it's only at that point that you determine to apply for the necessary partnership visa - that's the first point at which INZ need to be aware of a relationship. (It can equally well happen, and often does, that someone with no prior internet connection might meet a partner while on a WHV, and causes no surprise.)
Onward travel does not have to mean return to the home country. It can equally well mean travel to any other country where the person has right of entry.