Hello Jen and welcome to a2k
Having friends of the opposite sex is not so unusual, however,
your husband seems to be preoccupied with his "friend" a
tad too much.
If her intentions were honest she would send pictures and
any other correspondence to your house for you to see
as well. Not only that, she would make an effort to be friends
with you too, had she any interest in her friend's wife.
Dto. for your husband: had he any interest in his "friend's"
life, then he would encourage a friendship with her husband as well.
Both are interested in an exchange with each other only,
neither spouse is asked to join or participate. If it was me,
I would insist on being included in this friendship
while pursuing a more infrequent exchange at the same time.
If they both decline, than the answer is clear.