If your ENS used it as an attributive adjective, he's probably following a trend. He must have heard it somewhere - and he's a linguist, right? But 210 GHits for "funner evening". Not too encouraging.
http://tinyurl.com/6p2xagr

I asked my wife (USENS), I said: How about using funner as in "This is one of the funner evenings or one of the funner things to do" and she looked at me and said "sounds strange". Now, she's not a linguist but she is married to a guy who has asked her all kinds of linguistic questions. I said what if I really wanna use it. She said. Sure. So, I guess, it depends. As a lonely adverb, it really sounds weird: He's funner (no, I'd say "he's more fun") but maybe: she's the funnest gal.

How about:

1. We spent the funnest time at the zoo. ???
2. We had the most fun at the zoo. !!!!
3. The time at the zoo was the funnest (one). ?????????

If you can use it at all, it might depend on the context and if we're talking about people or things. And, it's seems to sound less crazy when used as an attributive adjective than as an adverb; if used as an adverb, the superlative seems a bit stranger than the comparative.

Like one of the comments in Juan's second link:
I don't care if "funner" is now in the dictionary, it is not a word! It sounds ridiculous and makes the speaker sound like a fool."

I probably wouldn't use it at a formal function or to get a job at the local newspaper, writing, that is. Actually, as an adverb, I wouldn't use it at all, because somebody's going to say: And you're from....?

:)

--------------------------------------------------
Note added at 2 hrs (2012-05-31 02:16:25 GMT)
--------------------------------------------------

typo corr: And, it seems to sound less crazy ...

--------------------------------------------------
Note added at 3 hrs (2012-05-31 02:30:13 GMT)
--------------------------------------------------

I could get used to " the funnest evening" (2,860 GHits): http://tinyurl.com/6s4ecbb

--------------------------------------------------
Note added at 3 hrs (2012-05-31 02:43:18 GMT)
--------------------------------------------------

compare to:
tan (adjective)
http://www.thefreedictionary.com/tan

If you're a fan of Fowler, here's what the 2008 Pocket edition of Fowler's Modern English Usage has to say:

fun, first recorded in 1700 and stigmatized by Dr Johnson as a 'low cant word' (i.e. ephemeral jargon), has long hovered on the brink of adjectival status (It was really fun) and more recently has taken a step further in informal attributive uses such as We had a fun time or That would be a fun thing to do. It still has a way to go, however, since it cannot yet be qualified by intensifying adverbs such as very or extremely (use of great instead gives away the noun's disguise). It also lacks the comparative and superlative inflections that a single-syllable adjective normally has, although more fun as in This sounds more fun is legitimate; funner and funnest have appeared in youth slang in American English and are now found in British English too, albeit usually in a jocular form:
She is also looking forward to the camaraderie of her country's locker room. 'Teams,' she said, 'are so much funner.'—Guardian, 2004.

That is a revised version of what was said in Fowler's Modern English Usage 2004:

fun. This modernish noun (first recorded in 1700 and stigmatized by Johnson as a 'low cant word') has become an informal quasi-adj., esp. in the second half of the 20C. We had a fun time, exclaims many a young person after a party, an outing, a holiday, etc., or It was a fun thing to do, meaning 'an amusing or enjoyable thing'. But it has not yet gained admission to the standard class of adjectives in that, in serious writing, it (so far) lacks a comparative and a superlative. In ordinary attributive use fun is quite frequent, esp. in funfair, the American word funfest (a gathering for the purpose of amusement), and fun run (an invention of the 1970s).