The hotel room and car rental are a non-issue. Those expenses would have been incurred anyway, even with a single professor attending. That there was another body in the car and in the room that were already going to be paid for anyway won't bother anyone - even the funding agency.
Without seeing the exact wording of the specific grant in question, no one can answer specific concerns. However with that said, most federal grants work along guidelines that sets amounts either in aggregate or in specifics for meals, or both - (just for example: $6 breakfast, $9 lunch, $15 dinner is pretty standard, up to a total amount of $1,500 for the grant period). So long as expenses are within those limits, no one is going to ask if some of the money paid for food that someone else ate. Ethically, yes, maybe even legally yes, it violates the spirit of, if not the legalities of, the grant contract. However, realistically, if the expenses stay within the limits as outlined, the probability that anyone will try to prove (or CARE enough to go to the expense and time of trying to prove) how much of what meals she ate is about zero.
Unless grant monies were used to purchase airfare, again it'll be a non-issue. Frequent flier miles and similar incentive programs aren't part of the budget, so their use generally isn't even considered. If grant money was used to pay for two tickets, and only one person attended on behalf of the agency, that's up to the person signing off on the expenses to catch. If they pass it through, the feds are unlikely to question it (although it is possible). If it IS questioned, it'll be the expense approver who gets in hot water as the custodian of the monies in question. People frequently aren't sure of what they'll get reimbursed for or not, so they submit everything under the sun, but ultimately, it's the person who APPROVES the expenses who bears responsibility to the agency and to the federal funding program. Unless there is some proof that the person submitting the expenses is in collusion with the person who approves the payment and/or reimbursement, with the intent to defraud, criminal charges aren't going to happen. At worst, the agency would have to reimburse the feds for anything that can't be substantiated.
And finally, unless you're the person auditing or approving the check that does the reimbursement, you can't assume WHAT he's being reimbursed FOR. It's common, for example, for spouses (or lovers, hookers, whatever) to travel together, eat together, etc. When the expenses are submitted on a $50 dinner tab, the grant will limit the amount of reimbursement to (for example) the $15 limit for dinner. So it doesn't really matter WHAT gets submitted - there are very definite guidelines and limits on what actually gets PAID.

