So finally we have some solid answers. Andrew it looks like your girlfriend will be good to use a calculator in around 5 out 7 casinos/cardrooms, based on the feedback from others, in Europe, cardroom Managers will be more than happy to accommodate with the ever powerful European Act on Human Rights and the UK Disability and Equality Act 2010. Stateside, the ADA governance and ruling of the supreme court of the Casey Martin v PGA Tour does, IN MY OPINION support your girlfriend in using a calculator, and if I was challenged with allowing your girlfriend to use her calculator at my tables, I would cite this supporting information as reasonable grounds to allow the use, whilst it was created in another country, I believe it's reasoning and judgment are solid enough to be used wordwide, and I think a European Court would rule for Casey just the same.
"she just needs help counting up the chips to equal the relevant bet" could be taken in multiple contexts, I chose a different interpretation of that piece of information, which was confirmed by Andrew a posts later in that I said "I think what Andrew is aiming at is that his partner has difficulty through no fault of her own at counting the pot, therefore if she has a calculator she can quickly surmise a rough pot count in order that she bets and raises amounts which are proportional." which was replied to by Andrew "Thank you Stuart, and yes she does have difficulty" which affirmed my interpretation of what he was wishing to convey.
I know one thing, if you speak the the majority of TD's who are reasonable, open and worth their weight they will have no problem letting your girlfriend sit at the table, calculator in hand. Don't let her be put off playing the game by TD's who are flippant towards her needs, and more so towards legislation designed exactly for these situations.
Regards
Stuart