I did the reverse for a while - played on an EU server to be with dutch and british friends. I had a couple of problems, which (plus my friends getting bored with RoM) is why I'm now on a US server.
1) Time zones -events tend to be scheduled based on local day time, and in particular siege wars.
2) Diamonds. I had to purchase with a credit card, NOT using the locally available cards, coupons, etc. But then because of fraud concerns (scammers using stolen credit cards) I wasn't allowed to gift items to others - including my own second account. I'd hoped to be able to make some gold selling diamonds, so I was somewhat unhappy. [Note that I don't believe the US market has the same restrictions]
Finally one more annoyance - the main website kindly redirects one to the "appropriate" local web site. Every time I wanted to check on current specials or similar, I wound up on the US web site, and had to click the British flag to get back to EU English. I tried saving a bookmark, but the redirect overrode it. I presume your British friend would have the same problem in reverse.