Your description sounds like he wants to use his account to purchase stock of the plan sponsor. Does the plan allow that? Who would the stock be purchased from? How would the purchase price be determined? I can see some possible PT issues.
As for the 5500 filing, a plan that holds non-publicly traded securities or employer securities cannot file Form 5500-SF. The SF filing is only available for small plans that are 100% invested in assets with a readily determinable market value and do not hold employer securities. Non-publicly traded assets can also affect whether or not the plan qualifies for the small plan audit waiver.