15 month old dog still toileting indoors

[Susieblue18]

Looking for some advice for my sister. Her small dog is a coton de tulear and is still doing the toilet in the house at least once a day. She takes her out regularly but at times she can be outside for a while then will come in and do the toilet (pee or poo!) when she gets in. Other times she will just go rather than waiting till she’s taken out. She can go all night in her crate without needing to go. She gets lots of praise and a treat when she goes outside. Any ideas for sorting this out? Thanks

Go back to basics and take it out every 30/60 minutes, and stay out until it goes. If the dog is eager to get back in, put a coat on it.
Has she cleaned where it toilets with the special cleaning product?

Go back to basics and treat the dog as a puppy. Outside every hour as well as after food, sleep, play and naps.

Ignore any accidents inside and just clean them up with an enzyme based cleaner,