While I cannot give you any special advice, there are sometimes also language issues. Many carriers actually demand you to speak some national languages, for example as far as I know, for working with LH
you must speak German, too, for SAS you must speak a Nordic language, and for other airlines you might also be required to speak the respective language additionally to English.
This does not apply to all LCCs, for example FR
has crews which only speak English, but I cannot recall that I have ever had a Non-German speaking pilot on any German LCC, for example...
Apart from that, I would guess that especially for the common types like A320 and 737, chances are good to get a job if you pass all requirements.
If he is a UK citizen, he can work in all EC
countries plus Norway, Iceland and Switzerland without any restrictions, so from a legal point of view, it is rather easy to work in any country in Europe. Whether this also applies to the respective license rules is something the others can answer better, but as far as I know, this is not a problem either...