I think I'm just going to rewrite the damn thing
It shouldn't be hard to improve upon...
-You can kill trees, cuz you hate trees... damn trees. Keep in mind this ability is only against non-sentient plant life. Also, you're useless if you don't do this. You're still useless when you do, but at least you and your party can hit each other with rotten sticks and feel badass because they break so easily.
-You can use a stock 1st level blasting spell as a supernatural ability, which you can then use to BURN TREES! Keep in mind you have to get within 10 feet to hurt anyone with this ability. You also don't have to eat or drink.
-You gain a version of wildshape that takes away a lot of the utility of wildshape. You are at least level 7.
-You can talk to roadkill. Specifically dead animals, who can't have intelligence scores higher than 2, so what the fuck do they have to say other than "Squirrel!"
-Another underwhelming Su ability that requires you to enter melee range.
-Now you can ANIMATE roadkill! You're moving up in the world!
-More uses per day of your officially shitty abilities. Which btw, are going up in x/day, which means even if they were good they would be useless.
-Kind of like sundering is a good NPC ability (NPCs sundering is devastating, PCs sundering is worthless), you now have the ability to separate people from their animal companions and shit. You spent at least 12 levels just so you could separate people from familiars and animal companions, which you should
just kill and animate, but you're not doing what you should do at 12th level. This would have been cool if they didn't condition out the experience penalty the target would take. Much like Sundering, his is an NPC trick, not a PC tool.
-See level 7
-If you get within 20 feet of an enemy you can potentially give them VD. Way to take the fun out of it dumbass. You're a level 15 character.
...that would be VERY easy to improve upon.
The blighter is obviously intended for NPCs, its fluff has nothing to do with PCs at all and its abilities are stupid-useless. Its spell progression is purely thematic. It even suggests NPC-like roles for the blighter under "Adaptation," calling them loners (aka not PCs, because PCs aren't loners no matter how many stupid fucking emo kids play D&D) or talking about secret cults (Also not PCs, PCs kick the shit out of secret cults on general principle, they don't join them.).
Rewriting an entire prestige class is a lot of work to do for one player's bad choice. It's a shitty class built on a stupid concept. You should redirect the player to something useful, because the blighter is just plain fucking awful.
If they're adamant, switching out the spell list (Make it Druid + All Necromancy) and throwing away the roadkill fetish could do a world of good for the class.