If it's somebody you barely know then they aren't a friend, but an acquaintance. There's an important difference here, I'd trust a friend to offer me a shell in good faith, but with an acquaintance I would not have a high level of trust.
That being said accepting a shell login from someone you don't know that well is a low risk to your system as there are few options there for him to attack you through your ssh client directly. The real risks come from being on the system itself:
- You don't know what he's doing. If he's engaging in criminal activity and you are logging into the same system as him you could end up being investigated
- Whatever you do on the system is available to him. If you write code or store personal information he could view it, if you use it for browsing he can see your history
- Downloads from the system could be used to attack you. This is the direct risk to your system.
How much of a risk this all adds up to be depends on how well you know this person and their reputation, what you plan to do with the offered shell, and what you have of value (are you a target). If you make sure your system and ssh client are all up to date and use good sense you shouldn't run into too many problems.
Just remember to practice good user ethics. this person has offered you something, don't abuse it by doing anything illegal or unethical with it. Also, take care not to mess with his server.