Is there a word for when you use a descriptor in place for a word/phrase you've forgotten?
Expressing one thing in terms of another, which is what your friend seems to have done, may be put under the umbrella of 'kenning', per the Oxford English Dictionary:
One of the periphrastic expressions used instead of the simple name of a thing, characteristic of Old Teutonic, and esp. Old Norse, poetry.
*Examples are oar-steed = ship, storm of swords = battle.
The term is adopted from the mediæval Icelandic treatises on poetics, and is derived from the idiomatic use of kenna við or til, ‘to name after’.*
Wikipedia has a very detailed and apparently scholarly article on Kennings, which I have linked to above. It isn't very quotable and I can't vouch for its accuracy at all, things are more simply put at YourDictionary.com:
A kenning is a figurative expression that replaces a name or a noun. Often it is a compound of two words and the words are hyphenated. Kennings are usually associated with Old Norse, Icelandic, and Anglo Saxon poetry.
Modern Examples of Kennings:
- Ankle biter = a very young child
- Bean counter = a CPA or accountant
- Bookworm = someone who reads a lot
- Fender bender = slight car accident
- Four-eyes = someone who wears glasses