A woman decides to sabotage her best friend’s wedding when she realizes her former fiance is proposing to her.

If it is her best friend, wouldn't she know that she is dating her ex-fiance? ?If this is the fiance quickly deciding to marry the friend instead of the protagonist, I think it needs to be more clear. Funny concept though!

Agreed with most of the above; I understand that the first "her" is the woman and the second "her" is the best friend, but it needs to be made more obvious.

You've got everything here it just needs a little tweaking, so i'll attempt a rewrite with my own spin:

A woman decides to sabotage a wedding when she?receives?two letters in the same day: a divorce notice and an invitation to the wedding of her best friend and?soon to be ex-husband.