Acquisition and summary
If you had previously untied Ashley's Top, and have not made up for it, Vickie can start Ashley's opportunity Redemption during her own opportunity Training Day. During this process, Derek can suggest that Ashley is a stoner, and would appreciate some pot (Opportunity The Candyman Can! discovered).
Alternately, after giving Ashley three Natty Lites as part of Drunk and Disorderly - Ashley, she will comment that it would be nice if you could bring her some "good-good" (Opportunity The Candyman Can! discovered).
To complete this opportunity, you need to find, acquire, prepare, then give some weed to Ashley.
You can ask everyone at the party whether they have any pot. Ultimately, you can ask Frank, who agrees to sell to you for the non-negotiable price of $50.
Obtaining Fifty Dollars
Derek can give you the idea that Stephanie is likely to have $50, as "her parents are loaded." To get the money from Stephanie, complete either her Doctor Stephanie opportunities (i.e.: Follow Your Calling, Apply Pressure, and Meltdown) or her Party Girl opportunities (i.e.: I'm Not Whipped, Bro! and Let's Lower Some Inhibitions), then ask her to dance topless.
When spoken to next, she will comment "I'm so fucking horny!", to which you can offer to help her with her horniness. She assumes you are offering sexual services as a prostitute. Confirm her assumption, then name your price of $50.
Strip naked, then select her and select Fuck. She will correct you, and you will start giving her cunnilingus. Complete the act, then ask her for the money. She will give you Fifty Dollars.
Alternately, while completing her Drunk and Disorderly opportunity, she will pass out in the Hot Tub. If you have spoken with Frank about buying his weed, while she is knocked out, you can loot Fifty Dollars from her.
Give the Fifty Dollars to Frank, and he will give you Marijuana in exchange.
Marijuana alternative method
Alternately, after you learn that Frank has the weed, you can loot it from him while he is knocked out. He can be knocked out either through direct combat, by Derek during his opportunity Derek Smash!, or he can concuss himself during Stephanie's opportunity Apply Pressure while trying to help Katherine.
Preparing weed for Ashley
At this time, if you simply offer the Marijuana to Ashley, she will comment that she can't use it. Your thought bubble will reveal "Guess I should figure out a way to get this into a more consumable form..."
To get the pot ready for Ashley, acquire the Paper from the desk in the Study Room. Use the Paper with the Marijuana to create a Joint (NOTE: This consumes the Paper; it will be unavailable to you if you have not already used it to get the code to Madison's Safe).
If the Joint is offered to Ashley, she will now ask for a light. You now need to light the Joint for her. This is done at the Fire Pit in the Backyard: Use the Joint with the Fire Pit to create Lit Joint. You can now give the Lit Joint to Ashley (Opportunity The Candyman Can! complete).
The opportunity name comes from song "The Candy Man" from the 1971 film Willy Wonka & the Chocolate Factory.
Ashley is an unapologetic stoner, going so far as to ask for weed through background chatter through the party. However, there is no smoking paraphernalia found in her room or elsewhere in the house, and she is not prepared to actually consume the weed when offered, even though the Player is able to with items around the house.
The inventory image for the Marijuana shows seven pot nuggets. This is likely enough to get the entire party high. It is unlikely that Frank would sell at a price of $50 for this quantity of weed, although he makes no comment on its quality, other than to say that if the Player has any complaints, that his HR director (i.e.: his fist) would be called in. It is further unlikely that the entire bag would be consumed in the creation of a single joint, unless the entire letter-sized paper was used for one massive joint. This could be why it had to be lit in the Fire Pit.
If Derek is mad at you when asking about Ashley, he can sarcastically give you purposefully misleading advice.