What's the joint sponsor's household size? Household size is spouse (if filing taxes Married Filing Jointly) + any dependants + anybody they sponsor or sponsored who is still not a citizen or hasn't earned 40 quarters of social security.
So let's say his income is 24K and his household size is 3 (sponsor + their spouse + immigrant they're sponsoring).
The 125% is $33312 as per https://www.uscis.gov/i-864p
Now the difference between $24000 and $33312 is $9312. To use assets, it must be 5 times the difference. 5 x $9312 = $46560
So $45k in bank would not be enough.
Also, you don't want somebody who barely qualifies. You need somebody who clearly qualifies. For example, somebody with household size of 3 and $50000+ active income, or better $80000-100000 in expensive areas like Bay Area, New York etc. Then adjudicator will be convinced they can sponsor somebody.