Hard Truths from a Best Friend

I must derail the rhapsody you’ve treated us to recently
about the new man in your life who’s treated you so decently.
Flowers or candy every date, then wining, dining, dancing.
No man in history ever excelled him in romancing.

It’s been such a whirlwind romance, developing so fast,
but I fear this sudden summer storm is one that will not last.
You may protest  I’m flaky and I don’t know what I’m saying,
but I would stake my life on the fact that he’s not staying.

What argument is plausible? What words can I relate
to convince you that your lover is a reprobate?
As your best friend, I must reveal this great love of your life
will fade away on the return of his vacationing wife!

Prompt words today are flaky,  rhapsody, derail, plausible and fast.