A Masjid close to our community had recently allowed a displaced Christian group to pray in their community hall. Is that acceptable in the Sharia?

Allowing Christians to pray in their community hall is okay, and it may be a good means of da'wah.[1] They should be invited to visit the mosque and meet Muslims to further enhance this da'wah opportunity.

Reference:

[1] This based on the fact that the muslim must be wise when calling to Islam and use the opportunities that come his way. Almighty Allah said: “Call to the way of your Lord with wisdom and fair admonition.  And argue with them in a way that is best." (Surah al-Nahl, verse 12)