Regarding the new online version of the deck-building card game Dominion, my friend complains that the bot is too challenging. Is there any way to decrease the difficulty when playing bots 1-on-1 in Dominion Online?
No, there is not currently any difficulty settings for the AI.
I cannot find an official source stating this to quote/link as reference, but you can read through the official FAQ and see some mentions of playing against the AI, with no mention of a difficulty setting.
As this is a pretty new implementation of the game, it is constantly in development with improvements and bug fixes happening often. So it is reasonable to expect such functionality to happen; though I have not heard about any expected timeline or priority for that feature. You can request that feature at the feature requests board on the Shuffle iT forums.