It's fine to translate to "friend" - as you said, the German phrase is also using the singular. It's just that in English you can't say "I have friend" - friend must be articulated; but you can say "I have friends", that is, friends in general. Likewise you can't say "I have other friend" nor "I do not have other friend". Since it's singular, it must always be "I have a friend" or "I do not have another friend" or "I have no friend". They also allowed translating to a plural "I do not have other friends" because it means the same thing and sounds very natural in English - perhaps they shouldn't have allowed it since perfectly fine translations exist that use the singular friend as in the original.