According to Gerry Beckley, the accepted author of Sister Goldenhair which debuted in 1975, he wrote the lyrics in the style of Jackson Brown and the title of the song was based on 3 of the group's mothers - all blondes.
Beckley takes sole responsibility and full credit for the lyrics so it would take some serious litigation to prove they came from anywhere or anyone else.
Given that your wife's ex is the one claiming to have written the lyrics and sold them it would be up to him to litigate and prove otherwise
Unfortunately, that the song may have been about your wife doesn't count for anything in the music or even other media world. If the subjects of songs were in line for compensation there would be an endless line of both provable claims and 'close, but not sure' claims. One famous example from the '70's is Carly Simon's "You're So Vain" reportedly about Warren Beatty. Beatty would never be in line for compensation because his name is never mentioned and nothing specific enough to link him exists.
Katy Perry, Taylor Swift are just two of countless current chart toppers who admit to writing about their ex's but again, no identities are clearly referred to enough to warrant legal action (or compensation)
Now, IF you can somehow prove beyond any shadow of any doubt that Beckley bought the lyrics and did not write them himself you likely have a collectible set of letters that with the right publicity can bring a good amount of money at auction
However, getting Beckley to admit to this will be a huge amount of work, starting with contacting the ex who sold the poem(s) and getting him to provide proof of that sale. And while the song came out in '75, the lyrics may have been kicking around for years (Beckley's most probable claim)
All in all, laying the groundwork and getting enough together to prove the former bf actually sold the lyrics to Beckley is just the beginning. The litigation thereafter to prove Beckley used what he bought and thereby has been dishonest in claiming he wrote them will be a long, very expensive road to travel
Without all of these obstacles overcome I'm afraid the lyrics and the poems are something special only to you and your wife.