There are several areas of passive fraud by being unclear on what you are doing.
When a citizen buys a house, the mortgage lender wants all the details as to how the buyer rounded up the money. That is so they can use their own formulas to assess the buyer's creditworthiness and the probability that the buyer will be able to keep up on payments, taxes and maintenance; or have they overextended themselves. The fraud is in the withholding of that info. By way of tricking them into making a favorable decision, when they might not have if they'd had all the facts.
Then there's making this sound all lovey-dovey, good intentions, no strings attached, no expectations. You're lying to yourself. What you've actually done is put money between yourselves, because you have not laid down FAIR rules to cover every possibility. You're not willing to plan for failure because you don't want to admit failure is possible, which is vain. Once you leap into this bell jar, the uncertainty of "what happens if..." will intrude itself into everyone's thoughts, slowly corroding your relationship. It's a recipe for disaster.
That uncertainty puts her in a very uncomfortable position. She has to labor to make sure the issue doesn't explode, so she's tiptoeing around you to avoid fights. Every fight, she'll wonder if you'll play the breakup card and threaten to demand the money back. The money will literally come between you. This is what money does. Thinking otherwise is a young person's mistake of inexperience. Don't take my word on it, contact Suze Orman and see what she says.
Your lender is also not going to like those poorly defined lovers' promises, because they've seen it all before, and don't want to yet again foreclose on a house that fighting lovers trashed. (it's like, superhero battles are awesome unless you own the building they trashed.)
This thing can still be done, but to remove this fraud of wishful thinking, you need to scrupulously plan for every possibility, agree to outcomes that are fair and achievable, put it in writing and share it with a neutral third party. You haven't done it, because it seems like it would be awkward as hell - and it will be! - Or it will test your relationship by forcing direct honesty about a bunch of things you haven't talked about or are afraid to - and it will! - And to be blunt, your relationship may not be able to survive that much honesty. But if it does, you'll be in much better shape.
The other passive fraud is taxes. By not defining the characteristics of the payment, you fog up the question of how your contribution will be taxed (if it will be taxed). A proper contract with each other will settle that. (there's an argument to be made for involving a tax advisor in the design of that contract, so that you can work things to your advantage.)
As an example, defining the payment as "rent" is about the worst you could do, as you will not be able to deduct any home expenses, she will need to pay income tax on the rent, but she can cannot take landlord's tax deductions on anything but the fraction of the house which is exclusively in your control; i.e. none.