Earlier, two guys were playing a casual match before our weekly store tourney. Duelist A attacked Duelist B's Set Blast Sphere with his Ehren, Lightsworn Monk. Both were confused on what would happen next. Duelist A said Ehren would return Blast Sphere to the deck while Duelist B said Blast Sphere would equip itself to Ehren. When asked, I agreed with Duelist A while saying I could be wrong and encouraging both players to look up the rulings for themselves, and after I've read over the rulings, I'm starting to agree with Duelist B since both effects are mandatory, and if memory serves me correctly, in this scenario, Blast Sphere's effect would be Chain Link 2 while Ehren's would be Chain Link 1. Still rather unsure, though, and an explanation backed by rulings and game mechanics would be nice to confirm or correct my assumptions on the matter.

The cards involved:

Ehren, Lightsworn Monk
Effect Monster
LIGHT / Warrior / 4 Stars
ATK: 1600 DEF: 1000
Passcode: 44178886

If this card attacks a Defense Position monster, return that monster to its owner's Deck before damage calculation. During each of your End Phases, send the top 3 cards of your Deck to the Graveyard.

Blast Sphere
Effect Monster
DARK / Machine / 4 Stars
ATK: 1400 DEF: 1400
Passcode: 26302522

If your opponent's monster attacks this face-down Defense Position card, this card becomes an Equip Card equipped to the attacking monster, without applying damage calculation. Destroy the equipped monster and this card during the Standby Phase of your opponent's next turn, and inflict damage to your opponent equal to the equipped monster's ATK.

So, who was right in this situation, Duelist A or Duelist B?

You were correct.

Ehren and Blast Sphere's effect would attempt to activate at the same point of time.

Ehren would activate first as it is the turn player's effect, then Blast Sphere as Chain Link 2, being the opponent's, and both are mandatory.

Blast Sphere would resolve first and equip itself to Ehren.

Ehren would attempt to resolve, but Blast Sphere is no longer present to be returned to the Deck, thus would resolve without effect.