"Mairi and a friend."
Translation:Màiri agus caraid.
caraid is lenited to charaid and preceded by a (a charaid) - i.e. it is put into the vocative case - when you are addressing the person directly, and is used more frequently as a term of address in Gaelic than 'friend' is in English (there are more situations in which caraid is lenited to charaid, but Duolingo has not introduced them at this stage, so I won't confuse you with them here). So if, as here, you are talking about a friend you would use caraid. If you are addressing someone directly as "friend", you would use a charaid. The vocative case is explained in the tips for the Phrases unit, or here in the combined tips: https://duome.eu/tips/en/gd