What are pros and cons for using summer-break to work in another research group?
Here are the obvious things:


Pro: the work in another group will build contacts, possibly improving your opportunities for future academic positions (PhD, postdoc, assistant professor position). 
Pro: funding opportunities, if you're in need of extra money.
Con: the work done in another group will take up time and energy, and if not related to your current work, won't advance your research (experimenting, publishing, etc.). That should probably be your priority (this is related to the comment on the question about it being strange to get 12-week breaks...)
Con: if you don't coordinate it with your supervisor, this could affect the trust in the relationship (depending on the style of the supervisor).


If you wish to pursue an academic career, in some universities respectable journal articles published are worth more than contacts when competing for a job. So don't overestimate the first "pro" I mention above. At the master's level, this is less important.

The best strategy is to coordinate the "outside" work with your supervisor for maximum benefit.