I comforted my gf but I also discussed my concerns and told her I don’t want Ashley helping to decorate our house anymore. I also told her that in the future, if I’m not consulted about plans, I’ll decline them and that I will be assertive with Ashley in the future.
I figure I can’t really decide how my gf chooses to deal with the situation, but when it negatively impacts me, I have to step in.
Sounds like you're doing well so far. When you're sharing concerns, stick to "I" statements and avoid being critical of 'Ashley.' That usually leads to the loved one getting defensive of their family member.
I had to come to terms that my H and his BPD mother will always want to feel close. The more I tried to convince him that their relationship was unhealthy, the more damage I caused. I eventually learned to quit convincing him to see it differently.
Set boundaries for yourself and give your gf the space to set her own boundaries if and when she's ready. Their relationship has been this way for many years so your gf will understandably feel more conflicted than you do.
You're in a great position to influence better boundary setting but it will likely be most effective to model it. Check out the info we have here on triangulation, it was probably the most helpful tool to help me find balance as I navigated my husband's relationship with his mom.
https://bpdfamily.com/content/karpman-drama-triangle