To be fair Frenchie did not say "congratulate" but "just as Christians should wish their brother and sister Muslims well during Ramadan" We Muslims do wish each other for a Ramadan accepted by Allah. We also pray for each others ease in the month. In short we wish well for each other.
On that note,there's absolutely no harm in a Christian wishing us well, specially if it brings about peace and harmony between us which is what we as Muslims should want..Peace!
On a lighter note I know many people have actually converted to Islam having come to see the generosity and kindness of many Muslims especially during Ramadan..Allahu Akbar!