what is this witchcraft candle used for?
i went to my bf house and i saw his mother had a black candle lit with a picture of death on it also she had a figurine of death holding a crystal ball and she i guess a offering to the santa muerte (saint death) an orange and apple

There is not a universal table of conversions within Spellcraft. Spells are about a persons intent and the objects they use within the ritual depends on the assignments or intents they give to these things.
A black candle can be used to summon death or keep death away. It can be used to summon negativity or keep it away. A pagan using Spellcraft would not be working with Saints, so I’ve got no idea what you’re talking about, but let’s make an assumption you saw the Death card from a tarot deck. This card could represent the same thing the black candle does, summoning or keeping away death. But it could also represent the end of something, such as a relationship, bad luck or a tough situation that someone is going through. It be something as simple as the end of a chapter in ones life, letting it go so they can move on to the next opportunities.
The figurine could be an object to help her focus on her intent during the ritual; much like a Christian would have a figurine of Jesus on an altar to pray to.
The orange and apple could be a variety of things and way too many assumptions to list. But an orange could be for protection (think vitamin C), it could be regenerating energy (think of the instant energy boost one gets from the natural sugar in an orange). The apple could be for health (an apple a day keeps the doctor away). It could represent truth (the forbidden fruit).
I’m curious why you didn’t ask her what the altar was for and respect her enough to give her the benefit of the doubt and opportunity to explain to you what her ritual was for. Could be she has a very sick relative that she’s very concerned about and is trying to bring healing to that person, and keep death from their door.
It’s easy for people to assume the worst, especially when they can’t be open minded enough to understand that not all Spell craft is negative. In fact the majority of it, isn’t.
For witchcraft?
Saint Death?
It means that the mom or/and the whole family is practicing Satanism. It is the worship of the devil to get whatever you want, and in some cases it actually works. But you should NEVER use it, because it can harm your mental state and the mental state of the ones you love. Talk to your boyfriend about it, or his mom if he won’t or can’t say anything.
Chesse?
She could just be a devotee of La Santísima Muerte. It might not actually "be" for anything; it could just be an offering to Santa Muerte.
And it’s NOT Satanism.
That sounds more like Vaudun (Voodoo)
Perhaps she was trying to appease death so he wouldn’t visit them.
It means that woman is full of superstitions and I do hope your boyfriend has not been contaminated by it.
And my guess is they are Roman Catholics, too.. so many of them from "old world" countries are really full of the pagan stuff still… I remember my old world European Roman CAtholic grand parents…. so Catholic but so filled with superstitions.