Thanks for your question and your thoughts and concerns about this situation. Your instincts are on track. There is a lot of research and conversation out today about just what you are intuiting; that tickling can be a very uncomfortable situation for little children, and yet, because of the laughter, there can be confusion for both parties as to whether it is "fun."
In the final analysis, some people feel it is best if there is no ticking, ever.
Others feel if the tickling is sweet and light, and the tickler will clearly stop if there is any indication that is wanted, then it can be okay.
Others advise always having a clear "stop" sign as a safety word, either a single word or a physical sign, like tapping the knee or something similar.
If the ticker doesn't stop, tickles rough, tickles against the will of the child or tickles as an excuse to touch the young person, it is considered abuse.
The fact that you feel uncomfortable about it is enough to insist that it stop.
Abuse in all forms more often happens from within the family, extended family and community.
All the best,