First, let’s understand exactly what your friend is doing. She is repeatedly soliciting friends and family – in, presumably, a form e-letter – for funds for her missionary endeavors … and without regard for the religious convictions of those to whom she is sending her solicitations. She is exploiting her personal relationships with friends and family to embarrass them into supporting something they may not otherwise dream of supporting. Given that, you would be justified to treat such solicitations like other spam you receive from charities you do not wish to support and delete them without response.
However, this is your friend and we should charitably presume that it has not occurred to her that her solicitations are in poor taste and could be considered offensive by those who do not share her religious beliefs. Rather than engage in an argument with her over her plans to evangelize a Catholic country, the charitable thing to do would be to explain to her that she may be straining relationships with other friends and family who are less tolerant than you of her behavior. If this results in her having to stay at home and earn her own way for her trips, then you will have accomplished the task of stopping her evangelism on this trip to a Catholic country without having to mention it at all. If not, at least you will have demonstrated more respect for her religious convictions than she has demonstrated for yours.
If at all possible, don’t write. Call or meet her. A person-to-person contact has more chance of persuasion. Whatever means you choose, I suggest saying something like this, as gently as possible:
I just received your letter and wanted to talk to you about it. I’m sure you did not intend it this way, but the letter is an impersonal solicitation. I know that you only wanted help in funding your mission work, but it is awkward for me as a Catholic to be asked – without regard to my personal convictions – to sponsor your work simply because we are dear friends. Since we are such good friends, I know your intentions were only the best, but perhaps others on your mailing list do not know that and have been inadvertently embarrassed and hurt by your appeals.
How I Solved the Catholic Problem by Kristine Franklin