Thank you for your reply.
It seems to me that when he was previously in the UK he may have breached his visa conditions whilst working voluntarily in the UK. As a visitor you are not allowed to work voluntary or involuntary, paid or unpaid. By working in the UK he breached his visa conditions.
As a visitor you are allowed to remain in the UK for 6 months, it is then assumed that you need to return to your home country as after a 6months visit (or less) you have affairs in your home country which need to be attended to. The UK have put at least a 6months limit from your previous visit to the UK to no raise any suspicions regarding your motives whilst in the UK.
Once your friend arrived to the UK before at least a 6 months gap from his previous visit the UKBA will assume that he may be using his visit visa's for purposes other than visiting, therefore they have detained him at the port of entry to ascertain his reasons for returning so soon, specially if he does not have the funds to pay for such a visit to the uk so soon.
The fact he has got a girl friend in the UK is a good reason to visit, however, this does not grant him automatic right to entry. Although the UKBA will take into considerations his relationship, they will not put too much weight on this when deciding whether to return him to Mexico or not. They would state that his girlfriend could visit him in Mexico instead when he is removed.
Unfortunatley it does not seem that there are ample reasons against the deportation of your friend, other factors such as his finances, returning too soon etc weigh against him.
I'm sorry if this is not what you wanted to hear.
I hope this answers your question, if so kindly leave positive feedback.
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