A question for all you knowledgable people: Will engraving aluminum damage the laser?

A friend of mine works an 8,000 watt industrial laser at his business. He says polished stainless steel, glass and mirrors won't harm a laser, but engraving (not cutting) aluminum will cause the beam to go back thru the modulator on his machine and burn it out. He says it's not a "physical" property of the aluminum, rather a "metallic" property. He's not sure if the same holds true for our kinds of lasers, but suggested that if I do engrave aluminum that I tilt one end up slightly so the beam doesn't go directly back thru the lens.

I don't believe our tabletop size lasers have modulators and anodized aluminum engravables are sold for our purpose. Does anyone have any opinion on this subject?