Among all places in Indonesia, Bali is the most liberal. Where is your girlfriend from? She should know that it usually is not a problem in any of the big cities and tourist destinations. Some hotels don't accept unmarried couples, but that's it.
Can I stay in Bali with my boyfriend?
Although Bali is making efforts to stamp out badly behaved tourists, on the whole, the island is very lenient when it comes to unmarried couples. Sex outside of marriage has been illegal in Indonesia for decades. However, it was the case that charges could only be bought by an aggrieved spouse in the case of an affair.