Yesterday, ROBLOX was having server issues, so maybe the issues are still relevant and your friend can't join because of them.
You can always look at:
https://status.roblox.com/ if its not working.
Is this issue a fluke or a trend?
If it's a trend it can be due to the specs of your pc not being powerful enough.
However if your pc meets the requirements then it may be something else.
Maybe it can be because of an update. ROBLOX like any other program needs to update in order to function.
If the client does not need to update then it may be something else.
ROBLOX often likes to crash a lot, so it might just occur naturally.
What is the severity of the issue? is it just an uncommon coincidence? or is it something much much worse that makes the game unplayable?
Reply to this post with in-depth detail so I can fully understand the issue that you are having.