Trending @Cognizant; Need clarification

One of my friend got an offer in cognizant as a fresher and he is having another offer too but he is waiting for a joining letter for the joining letter of that "x" company . Can he join in the cognizant and work for a few months and then when he got the joining letter from that "x" company .He can switch to the company immediately?? Any issues or challenges will be there ?? Kindly pour your suggestions