This is from his rules of writing: "Never use the word 'then' as a *conjunction – we have 'and' for this purpose. Substituting 'then' is the lazy or tone-deaf writer's non-solution to the problem of too many 'ands' on the page."
I thought this was really intriguing when I first read it. It gets the writer out of the habit of describing the action using, "and then, and then, and then," which is a sloppy way to write.
Just please keep in mind that publishing style will not only differ from from publishing house to publishing house, but also from region to region, country to country.
For instance, the argument this author deals with (over comma splicing) is an old one. Some style guides are adamant you shouldn't ever comma splice, others don't.
E.g., if you're American and you're going by the Chicago manual of Style (which a fair few publishers do like work tailored toward), yes, comma splicing is to be avoided, but not the use of 'then'.
To CMOS, they're not saying don't use it all, just don't use it as a coordinator (which is what I suspect your guy is saying):
Non-coordinator use:
He went to the restaurant, and then he phoned his missus.
He went to the restaurant; then he phoned his missus.
He went to the restaurant. Then he phoned his missus.
Not.
He went to the retaurant, then he phoned his missus.
'and' is implied as missing with the comma, but as 'then' isn't a coordinating conjunction, you can't use it to coordinate' two independet clauses like the example above.
If it was:
He went to the restaurant, then phoned his missus.
(and alternative with 'and' reinserted): he went to restaurant and then phoned his missus
'then' works fine because you have no subject before 'phoned', taking away two independant clauses and you sequent two verbs 'went' and 'phoned'.
With grammar advice, it's good to check where a member is from and take into consideration the little differences in usage and style.
I'm English and personally hate style books, but I can see why some publishers use them.
@Kado if word is insisting the you use an 'and' it's probably because it 'thinks' using a comma to suggest the missing 'and' would be better written as:
He went to the restaurant and then phoned his girlfriend.
Word is happy if you insert 'and' after the comma: He went to the restaurant, and then phoned his girlfriend. And it bugs me that it does that because that comma shouldn't be there. So it may just be screwing it up a touch because it knows 'then' isn't a conjunction, and it needs an 'and', so it's asking for one, albeit with the help of a comma when it's not needed.
Or it's picking up a comma splice.