In terms of punishment from the court, the worst would be a fine; byelaw offences (whether for TfL or the surface railway) only have that available as a punishment, and while in theory Regulation of the Railway Act offences can lead to prison, in practice they don't. It's just possible that you and your friend could be charged with theft and/or fraud, but that's pretty unlikely to happen and even so I would expect a fine as the punishment.
Impact on jobs? That depends on what you do for a living. There are some jobs in the City where you are dealing with other people's money so a very high standard of trustworthiness is needed. If you have one of those jobs, you will be in trouble. But for most jobs, if you are good at it and your employer wants to keep you, then there probably won't be a problem. But don't try to hide a conviction from your employer: that would prove that not only did you do a stupid thing a while back that got you a fine, but you were still trying to get away with not being honest. More likely than not, that would get you sacked.
Staying in the UK? We don't know and we can't say. There used to be so many people out there exploiting people new to the UK that the law now says that only proper immigration advisers can talk about this sort of thing. We're not proper immigration advisers so we can't help - sorry!