Don't
worry Ah ma, this isn't very complicating, yet. Ok time for another
lengthy post.
I am going to divide this post into parts which will let you see
clearly why this isn't too complicating like it seemed to be. All
he needs is a time to reflect, ponder and consider carefully.

His problem is attacking him into 2 areas
I) External factors - ex-gf working same company.
II) Internal factors - he discovered this new-found liking.
Basically, I do not know what causes them to break-up in the first
place, but since you said she dump him (and you chose the word
'dump') therefore I presume it wasn't a good closure of the
previous relationship. Trying hard to forget is NATURAL and the
process of trying to leave an old relationship for a new
life.
When she worked in the same company - they spend much time
together. This triggers the feelings - chances are, its never
liking UNLESS he admits that this present gf is a substitution for
her. In love, we fell for a number of reasons - two of them were
found here - time spend and begining-similiarity.
Beginning similiarity is a love psychological
behavior of sourcing out characteristic that is similar to you and
together with long time spend - it attracts. This attraction is
very familiar because its the same vibes that draws them together
the previous time.
Now she wants him back, there is two things I would like to point
out:
I) She is the one who 'dump' him - now he is leading such good
life, he should go ponder why he should end this 'good life' he is
leading now for something he made him fall into misery and left.
She came into his life not when he was the lowest point, rather
when he was all-well and easy. Her present gf? During his road to
recovery - & this is nothing to do with the arguement of
responsibility and not love. Rather this is a fine example of
compatiability, suitability and such. 
II) He and her could get along so well without much conflicts is
because there is a colleague status. I am pretty sure (85%) - that
outside work, there is little similiarity and personality harmony.
If he quits the job (like what Bear has mentioned), and try dating
her, when the feeling dies, you understand why.
In love, outside responsibility, we need to know whom are better
off when together in a relationship with us. In simple, the
compatiability. I have one very good question to ask him:
HE SO SURE HIS FEELINGS TOWARDS HIS EX IS DEFINITELY
LOVE? I don't think he knows for sure - and he is
pressured subconsciously by her chasing as well. 
P.S: When you have more choices to choose from - you look not at
the facts and figure presented to you, rather you look at yourself
and ask what do you really want. 
Cheers