Per Diem allowance

[Ballistic]

I have a friend (UK resident) who is a field service engineer and regularly visits customers in Europe for trips of a few days or a week at a time.

He is curios as to whether he can claim per diem as a daily allowance as an alternative to the company credit card currently used for major expenditure and a cash float for incidental items.

Does anyone have any experience of claiming this allowance and will he be legible?

If the allowance can be claimed how does it work and who pays it?

[Ballistic]

Many thanks for the replies so far.
I'll suggest to my friend that maybe this is not such a good idea.