I know Snowflake Reader Accounts let users query databases shared with them but can they connect the reader account to a 3rd party BI or Data Analysis tool like Tableau as you can with a Full Account.
Yes, a reader account is a "full account" that only reads, thus they cannot create tables etc. But most BI tools only read.
Now maybe there is a more subtle question that you mean, which is some tooling might understand their target DB and user things like temp tables to do work, which a reader account does not allow, and thus you are possibly really asking.
Can someone confirm Tableau works against a reader account?
Which I cannot directly answer, we have customers that use our reader accounts, but I have not seen specific see Tableau usage.