How bad is it to eat once a day?

I don't think it matters as long as she is eating enough calories in her one or two meals. I used to eat like that for several years when I was younger and I was slim. After my bf moved in with me we have 3 meals a day and I gained a lot of weight.

to be honest on a weekend i find i do this because i have lazy days where i wake up about 11-12 then have one big lunch/dinner meal and thats it. but during the week im very strict on lunch breakfast and dinner with the odd snack in between!

Depends on what you mean by 'bad'.

Ever been down to your last dollar and payday was 3 days away? You know how you scrimp and cut corners until then? Well, your body does the same thing. If it doesn't know when it's going to get it's next deposit, it starts to slow down. It cuts corners in terms of energy, and the quality of 'non-essential' processes like skin tone, strong & healthy hair & nails, etc. Minor injuries may not heal as quickly or as well. Your body temperature may run low. And it may not be immediately noticeable - maybe you're running at 92% instead of a 100 - but it adds up. It may choose to slow your metabolism down and store excess energy as fat in case the famine is coming.

When you train your body that you're going to feed it regularly several times a day, it knows it has energy to do everything it needs to do, and it will splurge expending extra calories on things like replacing skin cells, keeping you warm, keeping you awake & alert.

The key is - if you used to eat once a day, you cannot consume the same size meals. You still need to eat the same amount of food, but in smaller quantities at each meal. Twelve hundred calories in one meal is 400 calories in three meals - but if you're used to eating 1200, you may feel like you're starving yourself by only eating 600, but if you do that 3x a day, you're actually taking in an extra 600 calories a day than before.