Belgium maintains several vowel length distinctions that are absent in most of France, as you've noticed: a and â are usually only distinguished by length and not quality, pairs like sotte and saute use both quality and length (I tend to mishear French speakers with a short but tense [o] in saute as saying sotte), some vowels have been lengthened by a lost s or some final consonants followed by a lost e (gîte, voûte and ride are long, agite, soute and Madrid are short, for example)
One of the sources of those long vowels are Və sequences as in amie, émue, roue, créée, lieue (and more rarely/more marginally, voie and arrivaient), which contrast with ami, ému, roux, créé or lieu.
Those length distinctions are also present in Switzerland and in the eastern band of France between Belgium and Switzerland, but not really in the Nord-PaCa region. In Belgium, they're strongest in the regions where Walloon was a substrate, and might be less consistent in the Picard and Brabançon-subtrate regions (so your friend from Mons (technically Picard but right next to the border with Walloon) might do them a bit less consistently than someone from Charleroi, and once you get to Tournai, they just don't make most of those distinctions, just like someone from Lilles*)
*Another word with a long vowel, by the way.
As for the realisation, it's mostly just a normal Belgian short /e/(so [e̝], noticeably tenser than the cardinal vowel) but held for twice as long, maybe three times at most (200-250 ms for /e:/ vs 80-100 ms for /e/). Some speakers might have a noticeable low amplitude diphthong instead, something like [e̝ˑɪ̯], but the monophthong is the prestigious variant.
It's not the vowel of aise /ɛ:z/ which is long but open. Some speakers do raise closed syllable /ɛː/ to [eː] however, but that's seen as very lower class (it's normal in loanwords from Dutch, English or Walloon however, as in Maelbeek /maːlbeːk/, mail /meːl/ or Donceel /dõseːl/, and can happen without nearly as much stigma when the vowel isn't in the stressed syllable, as in j'aiserai /ʒeːzre/ for me)
So for verbs like réguler, you can have up to 4 é-like endings:
réguler, régulé with /e/
régulais, régulait with /ɛ/
régulée with /eː/
régulaient with /ɛː/ (as I mentioned this one is marginal)
Speakers around Charleroi, but typically not elsewhere might have a five way distinction:
régulé with /e/ [e] (short and not quite as raised as the usual /e/)
réguler with /eˑ/ [e̝ˑ] (half long and slightly raised)
régulée with /eː/ [e̝ː] (long and slightly raised)
régulais, régulait with /ɛ/
régulaient with /ɛː/ (still marginal, might be absent even if the speaker distinguishes régulé and réguler)
If that's what your Carolo friend has, this might explain why you hear régulé as closer to aise.
