In the answer the translation exercise, number 10, 'fear Bhríd' is used.

B'fhéidir go bhfuil bean Pháidín agus fear Bhríd anseo.

That can't be right since 'fear' is masculine, or am I missing something?

I think you're thinking of the behavior of an adjective after a masculine noun - you're right, it is not lenited:

fear maith - a good man/husband (masc.)
bean mhaith - a good woman/wife (fem.)

But Brid is not an adjective, it is a proper noun (in this case, a name). Proper nouns following/ in a genitive relation to another noun are lenited, regardless of the gender of the first noun.

Another way of explaining this is that when something belongs to a person, that person's name is lenited to show possession:

fear Bhrid - Brid's husband (masc.)
bean Bhrid - Brid's wife (fem. - only possible in certain states of the U.S.!)

I hope this helps.

makes sense

go raibh maith agat.