"He has plenty of buddies, but only one friend."
Translation:Il a plein de copains, mais un seul ami.
"Uniquement" implies "one of a kind", exclusivity or restriction.
"Seulement" implies "no more than" or an exclusion of other options.
In many cases, they are used as synonyms, but if you think of the related adjectives, you will see the difference:
- his friend has a unique talent
- his friend has only one talent
Basically "unique/ly" is qualitative and "only" is more quantitative.
In this sentence, he is said to have one friend, not two or three, so "only" is preferable.