It *might* be powdery mildew, but he is probably right that it's spider mites but in any case the treatment is mostly the same.
Spider mites are often found on the underside of leaves and look something like this: https://www.growweedeasy.com/wp-content/uploads/2016/08/spider-mites-on-leaf.jpg
Possible treatments are foliar sprays of Neem Oil (4tsps/gallon), Baking Soda (2tbsp/gallon), Hydrogen Peroxide (1tsps/gallon of 35% H2O2), Milk (yes milk, 1:10 ratio to water) and wetting agents like SM-90 (better for Powdery Mildew).
Spider mites thrive in hot and dry environments, so check your temp. and make sure there is good airflow (they hate wind). If your humidity is 70% as you have listed under your grow conditions that's a bit too high, esp. with this spider mite infestation you seem to have. Generally speaking during veg. RH should be 40-60% and 30-50% during flower, just keep a close eye during flower for bud rot and reduce RH accordingly. Good luck !
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